Flux-switching machines have been identified as a possible option for aerospace primary power generation as they offer high torque capability and good efficiency. Flux-switching machines are similar to switched reluctance machines, which make them more suitable to high-speed operation than a surface mount permanent magnet machine.
A typical example of a flux-switching machine is shown in FIG. 1. It has a simply constructed, annular single-piece rotor 10 having a plurality of annularly arrayed rotor teeth 12 on its radially outer surface. Coaxial with and radially outside the rotor 10 is an annular stator 14, which comprises an annular array of alternate C-cores 16 and permanent magnets 18. The permanent magnets 18 are placed circumferentially between adjacent stator C-cores 16 and concentrated windings 20 are wound in a conventional manner around each tooth 22, where a tooth 22 comprises a permanent magnet 18 and the nearer leg of each of the two adjacent C-cores 16. The machine can be wound for conventional sinusoidal three-phase operation. There is a slot area 24 between the legs of each C-core 16.
As shown in the enlargement of FIG. 2, typically the angle β made by the thickness of each C-core 16 leg, the permanent magnet 18 pole-pitch α and the slot pitch λ of the slot area 24 are equal. This maximises the torque generated for a given size of machine. Typically, each of these angles is the same as the angle made by each rotor tooth 12.
One disadvantage of this type of flux-switching machine is that there is a smaller slot area 24 available than in equivalently sized conventional permanent magnet machines having surface mount magnets on the rotor. This means there is a relatively high current density in the windings 20, particularly in larger machines with high electric loading, which can increase the temperature of those windings 20 and consequently reduce the life of the machine and its component parts.
The present invention seeks to provide a flux-switching machine that seeks to address the aforementioned problem.